Pedestrian motion in narrow passages at high population density shows some similarities with
granular dynamics [1]}. Though all
pedestrians have individual trajectories and destinations, their motion is principally
influenced by repulsive interactions with other individuals at high population
densities. This can lead to self-organization  of pedestrian
dynamics [2]}. At very high density, pedestrians can
organize themselves in lines [3]}, which is similar to
segregation in granular media [4]}. The behavior of pedestrians passing bottlenecks such as corridors or doors is in some respect analogous to granular flow in an hourglass [5]}. The
collective motion has the characteristics of self-organized
flow patterns [2]}. Density is used as a key parameter, closely related to pedestrian dynamics [7]},
but kinetic stress, i.e. the product of density (average mass per volume) and velocity fluctuations, is considered as another key factor leading to velocity changes towards the
bottleneck [8]}. If pedestrians stay in a confined not too
wide space, the density at a bottleneck will surprisingly reduce [9]}. If there is an obstacle in front of the door, it can help alleviate the pressure in
the area near the door and decrease the probability of jamming [10]}, [11]}, [12]}, [13]}. However, an obstacle does not necessarily alter the pedestrian flow rate [14]}. Some studies point out the 'faster-is-slower' effect [15]}, [16]}, which means
that pedestrians would pass the bottleneck faster if they are under lower stress. In a simulation study, an ordered or 'crystalline' structure was proposed for faster egress dynamics [17]}.
